Circuit breakers are typically used throughout distribution systems, for example, at multiple locations between the source of power and a load. The circuit breakers monitor one or more of the distribution lines, and when a fault occurs (such as a current overload or a ground fault), one of the circuit breakers trips thus interrupting the fault and removing it from the rest of the distribution system.
However, when a circuit breaker trips, it may interrupt or close down several distribution lines in addition to the line with the fault. Consequently, it is desirable to isolate the fault and interrupt the system as close to the fault as possible. This way, more of the distribution system remains operational.
It is known in the art, in an attempt to isolate the fault, that it is desirable to place circuit breakers with different trip characteristics at strategic points in the distribution system. For example, it is known to use circuit breakers with at least one or more of the following trip characteristics: long-time delay tripping, short-time delay tripping, and instantaneous tripping. Long-time delay tripping applies when an overload current is slightly larger than the current rating of the circuit breaker. In these cases, the circuit breaker trips pursuant to the known I.sup.2 =K relationship. The exact delay time for long-time tripping varies depending on the amount of overload current, but is typically about 3.5 to 30 seconds when the overload is about at the calibration point of the breaker (which is usually about six times the circuit breaker's current transformer rating).
Short-time tripping applies when the overload current is substantially higher than the current rating of the breaker. If the overload current goes above a predetermined amount, and the current does not fall back below this amount within a predetermined period of time, the circuit breaker trips. Short time tripping delays are usually between 80 to 400 milliseconds.
Instantaneous tripping applies when the overload current reaches a value approaching that which can cause catastrophic damage to the breaker or other equipment. When the overload current reaches this predetermined value, the circuit breaker trips without any intentional delay, which is usually around three line cycles due to unavoidable mechanical delays.
By placing circuit breakers with faster tripping characteristics closer to the loads, faults can be better isolated. Consequently, it is known to coordinate circuit breakers throughout a distribution system by placing circuit breakers with faster-time protection characteristics near loads, circuit breakers with longer-time protection characteristics farther from the loads, and circuit breakers with intermediate-time protection characteristics at intermediate locations. Therefore, the circuit breakers that are closer to the loads are given the opportunity to clear the fault before a circuit breaker trips that is father from the load.
More particularly, in a distribution system, most circuit breakers, regardless of location in the system, employ long-time trip characteristics. Circuit breakers closest to the load usually have instantaneous-time trip characteristics as well. Circuit breakers near the service entrance might be configured with instantaneous or short-time trip characteristics, while circuit breakers near intermediate feeders may also have short-time trip characteristics. This coordinated system allows the circuit breaker closest to fault to trip first, which maintains power to as much of the system as possible.
Consequently, it is desirable that certain breakers do not employ instantaneous tripping. Otherwise, they could trip before the circuit breakers that are closer to the fault, which may disrupt more of the system than is necessary. On the other hand, there are certain instances when the lack of instantaneous protection may cause damage to a breaker, such as when a breaker is closed into a fault. As is well known in the art, open breakers that are closed into faults are particularly susceptible to damage in a short period of time. In other words, a circuit breaker can generally withstand a higher fault current when it is latched closed than when the breaker is closed into the fault.
In some instances, a breaker may open pursuant to its short or long-time tripping characteristics, and then be re-closed before the fault was repaired or removed. If such a breaker only has long-time or short-time tripping characteristics, it may not react quickly enough to prevent damage to the breaker when it is re-closed into the fault. Therefore, it is desirable that such circuit breakers react quickly or instantaneously when closed into faults, while reacting pursuant to normal short or long-time trip characteristics under normal conditions.
In an attempt to prevent damage to circuit breakers while maintaining power to as much of a distribution system as possible, some known devices use a series of interconnected circuit breakers that communicate with one another. Each circuit breaker in the system is equipped with instantaneous tripping characteristics. When one breaker trips, it then notifies the other breakers that it has tripped, thereby temporarily overriding the instantaneous trip characteristics of those breakers. After a time delay elapses, those circuit breakers re-implement their normal instantaneous trip characteristics.
This known system, however, is costly and complex, and it does not necessarily prevent a tripped circuit breaker from being closed into a fault. For example, if one circuit breaker trips in this system, it overrides the instantaneous trip characteristics of the other circuit breakers for a period of time. If a second circuit breaker had tripped concurrently, or trips in the interim pursuant to its short or long-term characteristics, it could be closed into a fault and damaged because its instantaneous trip characteristic was temporarily overridden. Moreover, since each breaker is equipped with instantaneous trip characteristics, there is an increased risk that any of these breakers could trip during a fault before the breaker closest to fault trips. This would interrupt power to more of the system than is necessary.
Therefore, it is desirable to provide an inexpensive and reliable circuit breaker that provides short and/or long time protection characteristics on a full time basis, that will trip instantaneously when closed into a fault, and that does not employ instantaneous trip protection on a full time basis.